There is known a control apparatus for a hybrid drive system including a main drive power source, a power distributing mechanism for distributing a drive force of the main drive power source to an electric generator and a wheel-side output shaft, and an electric motor connected through an automatic transmission to the wheel-side output shaft.
Patent Document 1 discloses an example of a control apparatus for such a hybrid drive system including an engine, a power distributing mechanism in the form of a planetary gear mechanism for distributing a drive force of the engine to a first electric motor operable to generate an electric energy, and to an output shaft, and a second electric motor connected to the output shaft through an automatic transmission which has selectable two gear positions. The control apparatus disclosed in this publication is configured to control the engine, first electric motor, second electric motor and automatic transmission so that the hybrid drive system generates a required drive force corresponding to a required torque calculated on the basis of an operation amount of an accelerator pedal and a running speed of a hybrid vehicle.
For controlling the operating states of the engine, first electric motor and second electric motor, in particular, a major portion of the drive force of the engine is mechanically transmitted to the output shaft by a differential function of the planetary gear mechanism, and the remaining portion of the drive force of the engine is converted into an electric energy by the first electric motor operated as an electric generator, which electric energy is supplied through an inverter to an electric-energy storage device and the second electric motor, so that the second electric motor is operated with the electric energy, to generate a drive force to be transmitted to the output shaft. For supplying the electric energy from the first electric motor to the electric-energy storage device and the second electric motor, the hybrid drive system has a so-called “electric path”. An amount of electric energy to be supplied to the electric-energy storage device is increased and reduced according to an amount of electric energy consumption by the second electric energy, that is, according to an output of the second electric motor.
The control apparatus for this type of hybrid drive system is configured to implement a so-called “iso-power shifting control” to control a speed ratio of the automatic transmission while the output of the second electric motor is held substantially constant with the amount of its electric energy consumption kept substantially constant to equalize an amount of charging and an amount of discharging of the electric energy of the electric-energy storage device, with each other for the purpose of reducing a load of the electric-energy storage device. Namely, an output torque of the second electric motor is changed according to a change of its operating speed during a shifting action of the automatic transmission, so that an output of the second electric motor determined by a product of its output torque and speed is held substantially constant.
On the other hand, Patent Document 2 discloses a control apparatus configured to reduce a commanded value of the output torque of an electric motor when the electric motor is placed in an overheating state in a high-load running state of the vehicle, for example, for thereby limiting the output of the electric motor.    Patent Document 1: JP-2005-297948 A    Patent Document 2: JP-6-90507 A